1. Field of the Invention
The present invention relates to an optical data storage medium such as an optical disk on or from which information is optically written or read, by a laser beam, for example.
2. Discussion of the Prior Art
An example of a known optical data storage medium is shown generally at 60 in FIG. 7, which has a substrate 61 of an acrylic or polycarbonate resin, glass or the like, and a data storage layer 62 and a protective layer 63 which are formed on the substrate 61. The storage layer 62 may be a thin magnetic layer consisting of a metal or compound such as Te or TeOx, or GdTbFe, TbFeCo or other suitable magnetic material.
Information is written on the storage layer 62 by irradiating local spots of the layer 62 with a laser beam, and thereby heating the local spots, so that pits are formed selectively at the local spots, or the light reflectance or direction of magnetization at the local spots is changed or reversed. The information thus written on the storage layer 62 may be optically read, based on a variation in the amount of a read beam reflected from the storage layer 62, or by utilizing the magneto-optical effect, for example.
Usually, the substrate 61 has a spiral tracking groove or concentric tracking grooves, so that each recording track is defined by adjacent portions of the spiral groove or the adjacent concentric grooves. For accurate alignment of a read/write beam with the center of width of each recording track, the tracking groove or grooves is/are followed by a single tracking laser beam, such that the beam reflected by the medium 60 is detected by a push-pull or split photodetector. More specifically, if the tracking laser beam is centered on a certain recording track or between the adjacent two concentric tracking grooves, the two first-order diffraction beams are symmetrical with respect to the centerline of the recording track, and the differential signal produced by the split photodetector is zero. However, if the tracking beam is not centered on the recording track, the two first-order diffraction beams are no longer identical and interfere differently in the region of overlap with the zeroth-order diffraction beam, and the diffraction pattern is not symmetrical, whereby the differential signal from the split detector represents a positive or negative value, which can be used to center the beam right on the track. That is, a tracking servo control device is operated to position an optical read/write head (which generates a tracking beam as well as read/write beam) relative to the optical data storage medium, so that the differential output of the split photodetector is zeroed in a feedback manner.
In the presence of the tracking grooves formed in one of opposite major surfaces of the substrate 61, the data storage layer 62 and protective layer 63 formed on that one major surface are locally recessed corresponding to the pattern of the groove structure.
The push-pull or continuous far-field tracking method utilizing the diffraction beam pattern as described above tends to be easily influenced by disturbances such as vibrations, since the intensity of the diffraction beams is relatively small. Further, the shoulder or stepped portions between the recording tracks and the tracking grooves are likely to have different thicknesses. Since the anisotropic properties at the local recording spots are affected by the thickness of the recording tracks, the varying thickness at the edge portions of the recording tracks may lead to instability of the anisotropic information written on each track, causing a relatively low S/N ratio. Further, structural deterioration of the storage layer may arise from the shoulder portions.